Direct measurement of winding insulation does not provide a good indication of the moisture content of insulation in stationary induction apparatus, particularly those types immersed in oil for cooling purposes, and more particularly, with oil immersed induction apparatus with reduced insulation at the neutral. This is true because if the neutral insulation space is small, the stationary induction apparatus will have a large capacitance from winding to tank which substantially affects the determination of the dissipation or power factor. Thus, the condition of the insulation at the high voltage end of the winding, which is the important factor, will be masked by the large neutral end capacitance. This is especially true if high dissipation or power factor materials such as impregnated and laminated paper are used in the mechanical structure. In this arrangement when making field measurements, the power system must be disconnected from the induction apparatus because of the parallel capacitance of lightning arrestors, insulators, etc.
Another method to determine moisture content of stationary induction apparatus insulation is the measurement of the gas space dew point. The gas space dew point method to determine the moisture content requires a careful technique. Usually, electronic probes are used with the dew point method and these probes must be accurately calibrated and are also easily damaged. In addition, a long period of constant ambient and operating conditions is required to insure that there is equilibrium between the moisture content of the insulation, oil and gas space.
Still another method that is used to determine the moisture content of stationary induction apparatus insulation is the laboratory analysis of the moisture content of the oil. However, with this method, the sampling technique as well as the analysis is critical to a point that field personnel cannot normally perform the sampling nor the analyzing.
In addition, all of the above methods of determining the moisture content in stationary induction apparatus do not indicate the localized concentration of moisture in the insulation which is usually experienced in the lower or bottom portion of the oil tank.